1. Field of the Invention
This invention relates to a long distance discharge gap type spark plug comprising a center electrode, a porcelain insulator surrounding the center electrode and provided at its lower side with a firing portion made integral therewith and composed of a reduced diameter trunk wall, the firing portion extending up to near the lower end of the center electrode and exposing the lower end, and a grounded electrode opposed through the trunk wall of the firing portion to the center electrode.
2. Description of the Prior Art
Recently, as a countermeasure for exhaust gases, a thin mixed fuel has been vaporized, and as a result, it has been required to further improve the ignition property of spark plugs. It has been well known that the most effective means of improving the ignition property of the spark plug is to make a spark gap wide. In general, in a conventional spark plug adapted to ignite the fuel mixture in gaseous state, if the pressure in the combustion chamber of the internal combustion engine, which will hereinafter be called as applied pressure, is high, the requirement voltage becomes too high. As a result, it is impossible to make the spark gap long and hence to improve the ignition property.
In the conventional long distance discharge gap type spark plug, an attempt has been made to utilize a firing portion of the porcelain insulator surrounding the center electrode as a back electrode for the purpose of interrupting the spark discharge produced between the center electrode and the grounded electrode and hence of reducing the discharge voltage.
Such kind of the spark plug, however, has the drawback that if a surface gap formed between the front end of the center electrode and the grounded electrode is made long irrespective of the dimension of a minor air gap formed between the outer peripheral surface of the firing portion and the grounded electrode and of a thickness of the trunk wall of the firing portion, the discharge voltage becomes high, thereby inducing a puncture failure extending through the firing portion of the porcelain insulator. As a result, no occurrence of spark discharge is involved along the surface gap and hence degrading the above mentioned back electrode effect.